1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and more particularly to an electrophotographic photoreceptor producing less defective images such as background fouling, and having good environmental stability and uniform potential, and to an image forming apparatus using the electrophotographic photoreceptor.
2. Discussion of the Related Art
Recently, information processing systems using electrophotography are noticeably developed. In particular, laser printers and digital copiers converting information into digital signals with light have been dramatically improved in print qualities and reliability. The laser printers and digital copiers rapidly prevailing are being required to produce full-color images and images having higher quality.
Electrophotographic photoreceptors using organic photosensitive materials are widely used because of their cost, productivity, environmental stability, etc. The electrophotographic photoreceptors are broadly classified into functionally-separated multilayer photoreceptors including a charge generation layer having charge generatability and a charge transport layer having charge transportability, and single-layered photoreceptors having charge generatability and charge transportability in a layer. The multilayer photoreceptors are mostly used because materials thereof can be selected from a wide range, and sensitivity, stability and mechanical strength thereof have improved.
The charge generation materials for use in the charge generation layer of the multilayer organic photoreceptors include various materials such as azo pigments and phthalocyanine pigments. Particularly, phthalocyanine pigments having high sensitivity for light having a long wavelength of from 500 to 800 nm are most effectively used as a material for photoreceptors of electrophotographic printers and digital copiers using a LED and a LD as a light source.
Phthalocyanine includes a titanyl phthalocyanine pigment, a metal-free phthalocyanine pigment, a hydroxy gallium phthalocyanine pigment, etc. Specific examples of the titanyl phthalocyanine pigment include an α-type disclosed in Japanese published unexamined publication No. 61-239248, a Y-type disclosed in Japanese published unexamined publication No. 64-17066, an I-type disclosed in Japanese published unexamined publication No. 61-109056, an A-type disclosed in Japanese published unexamined publication No. 62-67094, a B-type disclosed in Japanese published unexamined publications Nos. 63-364 and 63-366, a B-type disclosed in Japanese published unexamined publication No. 2005-15682, a m-type disclosed in Japanese published unexamined publication No. 63-198067, a semi-amorphous type disclosed in Japanese published unexamined publication No. 1-123868, etc. Specific example of the metal-free phthalocyanine pigment include a X-type metal-free phthalocyanine disclosed in U.S. Pat. No. 3,357,989, a τ-type metal-free phthalocyanine disclosed in Japanese published unexamined publication No. 58-182639, etc. Specific examples of the hydroxy gallium pigment are disclosed in Japanese published unexamined publications Nos. 5-263007 and 5-279591.
The charge generation layer (CGL) is typically formed by coating a CGL coating liquid in which a charge generation material (CGM) is dispersed in a binder resin, and therefore it is essential that the coating liquid has dispersion stability to form a uniform CGL and the binder resin is known to affect the dispersion stability.
Japanese published unexamined publications Nos. 11-140337, 2007-219257, 2007-212670, 2006-133701, etc. disclose using a polyvinylacetal resin to stabilize the dispersion of a titanylphthalocyanine pigment. Japanese published unexamined publication No. 09-120167, etc. disclose using a polycarbonate resin. Japanese patent No. 3,016,296 discloses using a mixture of a vinylchloride-vinylacetate copolymer and a polyvinylacetal resin.
Photoreceptors using titanylphthalocyanine often have environmental stability problems because titanylphthalocyanine varies in sensitivity with humidity.
Japanese published unexamined publications Nos. 07-072638, 07-072637, 2006-154049, etc. disclose hydrophobizing a resin in a CGL to decrease the sensitivity variation due to humidity change.
The multilayer photoreceptor is formed by dipping a drum in a coating liquid to sequentially form a CGL, a charge transport layer (CTL), etc. on the drum. When an underlayer of the CTL is soluble with a CTL coating liquid, since the upper end and the bottom end of the drum are dipped in the CTL coating liquid for different times when coated therewith, the CGL is unevenly dissolved and the upper end and the bottom end of the drum have different sensitivity, and a charge transport material (CTM) penetrates the underlayer, resulting in background fouling.
Japanese published unexamined publication No. 08-160643 disclose cross-linking a polyvinylacetal resin and a melamine resin to form an underlayer of a CTL such that the underlayer is not dissolved by a CTL coating liquid.
Further, Japanese published unexamined application No. 6-83078 discloses, when a polar group derived from a monomer including a hydroxy group such as 2-hydorxyethylacrylate is introduced into a vinylchloride-acrylic polymer binder to reduce aggregation of a CGM for improving storage stability of a CGL coating liquid, an isocyanate compound may be reacted with the vinylchloride-acrylic polymer to break the hydroxy group deteriorating solvent resistance and humidity resistance of a photoreceptor. However, specific examples thereof are not disclosed and an azo pigment is used as a CGM, and does not aim at maintaining sensitivity and chargeability of a photoreceptor using titanylphthalocyanine. Japanese published unexamined application No. 7-72634 discloses in its Examples 8 and 16 a CGL including ω, ω′-bis(p-N,N-dialkylaminoaryl)-polyethylene oxide and formed with a coating liquid including polyvinylbutyral and tolyelen-2,2-diisocyanate as a binder for a photoreceptor having less irradiation fatigue and high sensitivity. However, this uses an azo pigment as a CGM as well, and does not aim at maintaining sensitivity and chargeability of a photoreceptor using titanylphthalocyanine, either.
Because of these reasons, a need exists for an electrophotographic photoreceptor preventing background fouling, having good environmental stability, uniform potential characteristic, and sufficient and stable high sensitivity.